In modern electrophotographic equipment, toner particles are automatically recycled many thousands of times over image surfaces while the particles are moving at an extremely high velocity. Further, toner particles, which are deposited in image configurations, must now be fused in extremely short periods of time. Thus, it is necessary for toner materials to possess the proper triboelectric charging properties for electrostatic latent image development and furthermore, they must not agglomerate during storage and transportation. Thus, it is necessary for the toner to endure the harsh environment of high speed electrostatographic copiers and duplicators and be capable of fusion at lower energy levels.
Polymers have been developed which exhibit properties meeting the stringent standards of advanced copiers and duplicators. An example of a copolymer used for a toner is a copolymer of styrene-butadiene. Such copolymers of styrene-butadiene may be made by various techniques such as solution and emulsion polymerization, however, suspension polymerization has been found to be the most suited for forming copolymers for use in toners. Toners may also be produced by other methods. For example, copending U.S. patent application Ser. No. 548,328, now U.S. Pat. No. 5,155,193, filed Jul. 2, 1990, "Suspension Free Polymerization Process and Toner Composition Thereof", which is incorporated in its entirety herein by reference, discloses a suspension free radical polymerization process.
The styrene-butadiene suspension polymerization process as described in U.S. Pat. No. 4,558,108 (the '108 patent), which is incorporated herein in its entirety by reference, uses a series of depressurizations/repressurizations ("venting") of the reactor headspace in order to reduce the residual butadiene in the polymer to levels which are environmentally acceptable. The '108 patent discloses a process for forming in a reaction vessel a copolymer of styrene and butadiene in which there is provided an aqueous suspension phase comprising water, styrene monomer, butadiene monomer, a suspension stabilizing agent, and a chain propagating amount of a free radical polymerization initiator which is insoluble in water, soluble in the styrene monomer, soluble in the butadiene monomer and having a 1 hour half-life between about 50.degree. C. and about 130.degree. C., the ratio of the styrene monomer to the butadiene monomer being between about 80:20 and about 95:5 by weight, the weight proportion of water to the combination of the styrene monomer and the butadiene monomer being between about 0.8:1 and about 2:1, and wherein the suspension stabilizing agent consists essentially of a finely-divided, difficultly water-soluble powder; and a vapor phase comprising an inert gas and butadiene monomer. In the process the aqueous phase and the vapor phase are heated to a temperature between about 50.degree. C. and about 130.degree. C. at a pressure between about 20 psi and about 140 psi until at least about 90 percent by weight of the styrene monomer and the butadiene monomer are copolymerized to form an aqueous suspension of discrete polymer particles having a Tg value of between about 45.degree. C. and about 65.degree. C., a weight average molecular weight of between about 10,000 and about 400,000, a molecular weight distribution of the copolymer between about 2 and about 9 and a butadiene monomer concentration of less than about 10 parts per million by weight. During the process, butadiene monomer is removed from the vapor phase, by venting and repressurization of the reactor vessel, after at least about 75 percent by weight of the butadiene monomer and the styrene monomer in the aqueous phase have been converted to a copolymer and prior to conversion of more than about 98 percent by weight of the butadiene monomer and the styrene monomer to a copolymer in the aqueous phase.
FIG. 1 represents the standard time/temperature/pressure profile of '108 patent venting process. The steps of the prior process are as follows: First, the reactants are charged in a reactor ("charge phase"; labelled "A" in FIG. 1). The reaction is initiated and continues exothermically ("exotherm phase"; labelled "B" in FIG. 1) with an initial increase in pressure and temperature, after which the temperature is maintained at a polymerization temperature of 95.degree. C. In the preferred embodiment of the '108 patent, about 165 minutes after the start of the exotherm phase, the reactor is vented ("venting phase"; labelled "C" in FIG. 1) with a concomitant decrease in pressure resulting in foam formation in the reactor. The vent is then closed and the pressure is increased until the foam disappears. This depressurization/repressurization venting procedure is repeated several times for approximately 30 minutes. The temperature of the reaction vessel is then raised over a 40 minute period ("heat-up phase"; labelled "D" in FIG. 1) to 125.degree. C. to complete the reaction process. After maintaining the high polymerization temperature for approximately 75 minutes ("high temperature phase"; labelled "E" in FIG. 1), the reactor is then cooled ("cool-down phase"; labelled "F" in FIG. 1) for approximately 100 minutes, thus bringing the total reaction time to approximately 400 minutes.
Although the method of the '108 patent produces polymer with an acceptable level of residual butadiene, the depressurization/repressurization venting procedure creates some inconvenient processing characteristics, such as:
1) the venting procedure delays the process about 30 minutes before the heat-up phase can begin;
2) foam generation during the depressurization cycles can create disturbances which adversely effect the efficiency of the process;
3) extensive above liquid level fouling of the reactor and reactor appurtenances due to foaming can take place. This is believed to be a major factor in necessitating frequent reactor cleaning.
Each of these characteristics of the '108 patent process increases the cost and decreases the efficiency of the polymerization process. Other processes for making these and similar types of polymers exhibit similar inconveniences.
It would therefore be desirable to provide a process for polymerization which produces acceptable levels of residual butadiene in the resultant polymer, while overcoming these inconveniences of prior methods.